Providing Shading in the Greenhouse
Don’t leave attention to shading until it is too late. You will find that correct shading, ventilation and damping down, are essential in the greenhouse for the cultivation of strong and vigorous plants.
Excessively high temperature in summer, coupled with lack of shading, is a common cause of failure among beginners in greenhouse and frame gardening. There is also still much misunderstanding about the subject, even where professional gardeners are concerned.
It is an advantage to site a greenhouse in an open sunny position, because this gives much free warmth in winter but, because of the heat-trapping effect of glass, shading will invariably be required in summer. Often it may be necessary much earlier than this during sunny periods, when temperatures can be sent rocketing, even in spring (February or March). Plants that are not used to bright sunlight and warmth, having just been through the winter, may wilt. The numerous greenhouse plants that flower in spring, such as cineraria, calceolaria and primula, will last much longer if kept shaded and cool. Seed sowings will also need shading at this time.
In summer an unshaded greenhouse, especially one that is badly ventilated and watered, may become an oven. Temperatures over 50°C (120°F) can be easily reached and even tropical plants will be shrivelled by the end of a day. Plastic greenhouses should be less of a problem in summer, because the solar heat is not trapped to such a great extent as with glass, but shading will still be necessary.
How much light?
A knowledge of the light requirements of the plants you grow is essential. In a mixed greenhouse the plants can then be placed accordingly. Deep-shade lovers, like many sub-tropical and house foliage plants, can go under the staging. Those preferring less can be sited in the natural shade of taller plants on the staging or under climbers. Plants demanding maximum light can go on shelves or under parts of the roof left unshaded or only slightly shaded during summer.
The effect of too much light and sun will be seen as bleached foliage, which in severe cases can become literally scorched brown. The plants will wilt, and if you have been watering them from above there may be brown spotting of the leaves. Droplets of water act as minute lenses, focusing the sun’s rays and causing these burned spots. Plants with hairy or furry foliage are particularly prone to this kind of damage because the water droplets are supported just above the leaf surface and the focus is then much better. Plants commonly affected are saintpaulia, gloxinia, smithiantha, and others of the gesneria family with hairy leaves.
High temperature dangers
Greenhouse fruits, such as the tomato, can become blistered by excessive sunlight. Brown to grey watery ‘scald’ marks appear on the fruits. In the case of tomatoes high temperature is a frequent cause of failure, and shading is nearly always important during the ripening stage. At temperatures over about 32°C (80°F) the red pigment of the tomato fails to form properly and the yellow persists. In addition there may be numerous other ripening faults, such as ‘greenback’, where the fruit is unevenly ripe and patched with red and yellow or green. Ventilation and white shading should be used to keep down temperatures during hot sunny weather.
The necessity of shading
Once the sun’s rays have penetrated the glass or plastic lights of a greenhouse or frame, a part of them is converted into heat after striking the interior. This heat cannot readily pass back through the glass and so is ‘trapped’ inside. It is obvious therefore that for shading to be most efficient it must stop the sun’s rays before they penetrate the glass. Exterior blinds are very effective, but remember that they also will absorb heat from the sun; so that this heat is not transferred to the glass or plastic roof the blinds should be a few inches above it. This will allow air to circulate and carry away heat.
Well-designed greenhouse blinds rest on runners and generally the slatted type of blind is best. These can be made of timber or of bamboo, and have to be made specially for your greenhouse and to the measurements you give the makers; they are lowered or raised by cords in the conventional manner. It is also possible for the blinds to be photo-electrically controlled, giving automatic operation.
Slatted blinds will of course let some direct sunlight pass through, but this light does not remain in one spot for long, owing to the sun’s movement across the sky. Hessian or plastic sheeting and other materials are also sometimes used as blinds, but are very easily blown away.
In recent years interior blinds have come onto the market. These types are convenient because they stay clean and are not in danger of blowing away. However, they are of little use in keeping down temperature.
For very many years shading paints, applied to the exterior glass, have been used as a cheap method of shading during summer. Various mixtures of flour, whiting,and chalk have been suggested — often messy and even damaging to structures. Serious accidents have occurred, such as putting an arm through the glass when trying to scrape off such mixtures at the end of the season. Proprietary preparations of this kind were often tinted green, and even some modern methods of shading still employ the colour green.
Research both in Britain and in Holland has shown that white is the best colour, as it reflects back a much wider range of the sun’s spectrum. Green actually absorbs much solar energy (indeed, this is the principle of photosynthesis of plants) and so a green-shaded greenhouse becomes considerably hotter than a white-shaded one. Plants also seem to grow better in a white-shaded house and there is a better rendering of flower and foliage colours as well as more sturdy growth. Remember that the shade cast by green leaves, such as in a wood or under shrubs, is not the same as that cast by a green-painted glass roof.
A remarkable modern invention is an electrostatic type of shading paint, which is marketed under the name of ‘Cool-glass’. This is specially formulated to reflect back as much of the sun’s heat as possible but to let useful rays through. It can be sprayed or brushed onto the glass and diluted to give any degree of shade needed. It has the curious property of staying firmly on the glass even during heavy rain, yet being easily wiped off with a dry duster. This type of shading has given very good results with crops like tomatoes, cucumbers, fuchsias, begonias, carnations, orchids, chrysanthemums, foliage plants, and most frame crops.
1. By placing your plants in different positions in the greenhouse, you can suit all their requirements for warmth and light. Small heat- and light-loving plants, such as Columnea gloriosa, will do well if placed on a high shelf.
2. Larger heat- and light-loving plants can be placed on the staging. Plants which will thrive in this position, and provide an attractive display, include gloxinias, regal and zonal pelargoniums, primulas, cinerarias, calceolarias and hydrangeas.
3. Some plants, especially many foliage plants, prefer shade; and those with variegated leaves, such as Tradescantia virginiana, can lose their colour if left in bright sunlight. The best place to keep them is underneath the staging.
4. An end wall is a good site for climbing plants. The east wall is the best place, in this greenhouse, for a cucumber vine. It likes a draught-free position — well away from the door — and plenty of heat and light without being in direct sun.
5. Some plants, like tomatoes, need plenty of heat and sun. The ideal place for them is along the south side of the greenhouse. However, even sun-loving plants need shading on very hot days when excessive temperatures can scorch their foliage.
6. Two important pieces of greenhouse equipment concerning heat and light; a propagator, for providing the high temperatures needed by cuttings and germinating seeds, and an exterior blind for the most effective shading.